The present invention relates to a variable compression ratio system for an internal combustion engine which is capable of continuously and variably controlling a compression ratio of the engine depending on engine operating conditions, and a method for controlling the system.
U.S. Pat. No. 6,491,003 (corresponding to Japanese Patent Application First Publication No. 2002-115571) discloses a variable compression ratio system for a reciprocating internal combustion engine. The variable compression ratio system uses a multiple-link type piston-crank mechanism for varying a position of a piston bottom dead center (BDC). The multiple-link type piston-crank mechanism includes upper and lower links linking a piston pin of a piston to a crankpin, and a control link linking the lower link to an eccentric cam of a control shaft. An actuator drives the control shaft to vary the rotational position depending on the engine operating conditions, whereby the compression ratio is variably controlled. The actuator may be an electric actuator, namely, an electric motor, or a hydraulic actuator.